U.S. Pat. No. 5,940,257 discloses a device for measuring multi-phase alternating currents, designed to allow the alternating currents to be measured in as precise a manner as possible so as to be able to reliably detect an earth fault in a power distribution system for multi-phase alternating current, and to thus be able to actuate a switch disconnector in an even more precise manner. The measuring device comprises a current transformer which converts an alternating current flowing through a line, which is to be monitored, of the distribution system into a measuring alternating current. The measuring alternating current is converted by means of a rectifier circuit into a direct current, which is amplified by an operational amplifier operated as a differential amplifier and fed to a controller for detecting an earth fault.
U.S. Pat. No. 4,672,501 describes a switching device in which voltage and current values are detected continuously and monitored by a processor in order to determine the electrical state of a current distribution system. In the event of an overload current or an undervoltage, the current distribution system is interrupted by the actuation of a circuit breaker. When the overload current state or undervoltage state has passed, the contacts of the circuit breaker can be closed by actuating a controller relay. The processor is also configured to execute a measurement algorithm which can carry out power-based measurements on the basis of in a digitized, unrectified measuring current. A similar circuit is known from EP 1 388 920 A2.